#include <cstdio>
#include <cstring>
using namespace std;

int v[1000];
char q[10050];

int main()
{

    freopen("litere.in","r",stdin);
    freopen("litere.out","w",stdout);

    int n, i, j, s, t, d;
    s = 0;

    scanf("%d%*c",&n);
    scanf("%s",q);

    for( i = 0; i < strlen(q); ++i ){
        for( j = q[i]+1; j <= 'z'; ++j )
            s += v[j];
        v[q[i]]++;
    }

    printf("%d",s);

    return 0;
}